The purpose of this study is to evaluate the analgesic efficacy and safety of a continuous subcutaneous infusion of sufentanil supplemented by patient-controlled analgesia (PCA) and to establish the dose ranges to be used in subjects with chronic pain. UCSF/Mt. Zion Pain Management Center (PMC) will be a participating center in this open-label, multicenter, non-randomized, dose-escalation study to determine the feasibility, safety, and effectiveness of analgesic therapy with a continuous, subcutaneous infusion of sufentanil. The study will have 2 parts. Part 1 will enroll a total of 10 chronic pain patients undergoing inpatient titration of the sufentanil infusion rate during 3 days. Part 2 will enroll a total of 40 patients undergoing the same inpatient titration followed by 1 week of outpatient therapy at the delivery rate established during the inpatient phase. We request that the study be conducted at the GCRC. The GCRC's utilization will ensure that efficacy measures are completed, safety is monitored, biological specimens are collected, prepared, and sent to central laboratories for analysis, and that data are captured on sources and transcribed into Case Report Forms which, in turn, will be collected and analyzed to prove the safety and efficacy of the drug/device.